The program objective is to facilitate the management of the NlH Animal Genetic Resource (NlHAGR) through development and utilization of techniques such as collection, culture, freezing and transfer of embryos and gametes for cryobanking genetic material and improving reproductive potential. The NlHAGR maintains a number of critical species and, within species, vast number of invaluable genotypes used in biomedical research. Embryo preservation uses a cryoprotectant in the freezing medium which allows sufficient dehydration to protect the blastomere during freezing. Following thawing and dilution of the cryoprotectant, embryos are cultured to assess in vitro development. Transplantation of embryos into recipient females monitors in vivo development and thus biological competence. In vitro fertilization and ova freezing are being examined as techniques to salvage limited genetic material from single females or strains with poor in vivo fertilization rates. Banked embryos are maintained in a frozen state until needed for rederivation. A primary focus of laboratory efforts is the study and optimization of cryobanking technology in mice and more recently, in rabbits. A major research finding is that embryo freezability is confounded and influenced markedly by mouse genotype. Such responses have been examined in over 55 genotypes, most of which display genotype specific survival rates to standardized embryo freeze thawing procedures. Ancillary projects are oriented toward the comparative aspects of improving long-term embryo storage with particular emphasis on the study of cryoprotectants and enhancing methods for embryo freezing, thawing, dilution and transfer. Efforts in the rabbit focus on improvements in collection procedures and preservation of embryos using a newly developed vitrification procedure.